1. Field of the Invention
The present invention relates to a magnetoresistive device that incorporates a magnetoresistive element and a method of manufacturing same, and to a thin-film magnetic head that incorporates a magnetoresistive element and a method of manufacturing same.
2. Description of the Related Art
With recent enhancements in the areal recording density of hard disk drives, improved performance has been sought of thin-film magnetic heads. For the past few years in particular, hard disk drives have been doubling in areal recording density roughly each year, requiring areal recording densities of 100 Gbit/(inch)2 or more lately.
Among the thin-film magnetic heads, widely used are composite thin-film magnetic heads made of a layered structure including a recording head having an induction-type electromagnetic transducer for writing and a reproducing head having a magnetoresistive element (that may be hereinafter called an MR element) for reading.
MR elements include: an AMR element that utilizes the anisotropic magnetoresistive effect; a GMR element that utilizes the giant magnetoresistive effect; and a TMR element that utilizes the tunnel magnetoresistive effect.
Reproducing heads that exhibit a high sensitivity and a high output are required. Reproducing heads that meet these requirements are GMR heads incorporating spin-valve GMR elements. Such GMR heads have been mass-produced.
In general, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces that face toward opposite directions; a free layer that is located adjacent to one of the surfaces of the nonmagnetic conductive layer and has a direction of magnetization that varies in response to a signal magnetic field from a recording medium; a pinned layer that is located adjacent to the other of the surfaces of the nonmagnetic conductive layer and has a fixed direction of magnetization; and an antiferromagnetic layer that is located adjacent to one of surfaces of the pinned layer that is farther from the nonmagnetic conductive layer and fixes the direction of magnetization of the pinned layer. The free layer and the pinned layer are each made of a ferromagnetic layer. An electric resistance value of the free layer varies according to the direction of magnetization of the free layer. The spin-valve GMR element utilizes the variations in the electric resistance value of the free layer to reproduce data that is magnetically recorded on the recording medium.
Another characteristic required for reproducing heads is a small Barkhausen noise. Barkhausen noise results from transition of a domain wall of a magnetic domain of an MR element. If Barkhausen noise occurs, an abrupt variation in output results, which induces a reduction in signal-to-noise ratio (S/N ratio) and an increase in error rate.
To reduce Barkhausen noise, a bias magnetic field in the longitudinal direction (that may be hereinafter called a longitudinal bias field) is applied to the MR element. To apply the longitudinal bias field to the MR element, bias field applying layers may be provided on both sides of the MR element, for example. Each of the bias field applying layers is made of a laminate of a ferromagnetic layer and an antiferromagnetic layer, or a permanent magnet, for example.
In a reproducing head in which bias field applying layers are provided on both sides of the MR element, in general, a pair of electrode layers for feeding a current used for signal detection (hereinafter called a sense current) to the MR element are located to touch the bias field applying layers.
In general, the MR element is sandwiched between a bottom shield layer and a top shield layer. A bottom shield gap film that is an insulating film is interposed between the MR element and the bottom shield layer. Similarly, a top shield gap film that is an insulating film is interposed between the MR element and the top shield layer. A base layer may be provided between the MR element and the bottom shield gap film for the purpose of attaining better orientation and magnetic properties of magnetic layers that constitute the MR element. For example, a Ta or Cr compound may be used as a material of the base layer. Between the MR element and the top shield gap film, a protection layer may be formed after forming films making up the MR element, for the purpose of protecting those films. The protection layer may be made of Ta, for example.
Reference is now made to FIG. 21 through FIG. 24 to describe an example of a method of manufacturing a reproducing head. In this manufacturing method, as shown in FIG. 21, a bottom shield gap film 104 made of alumina (Al2O3), for example, is first formed on a bottom shield layer 103 made of NiFe, for example. A base layer 105 is formed on the bottom shield gap film 104. Then, an MR-element-to-be film 106P to make the MR element is formed on the base layer 105. A protection layer 107 is then formed on the MR-element-to-be film 106P. Then, a mask 108 for patterning the MR-element-to-be film 106P by etching is formed on the protection layer 107. The mask 108 is made of a photoresist layer patterned by photolithography. For easy lift-off, the mask 108 is formed to have a T-shaped cross section, i.e., such a shape that a portion close to the bottom is smaller in width than a portion close to the top.
Next, as shown in FIG. 22, ion beam etching is performed so that ion beams travel at an angle of 5 to 10° with respect to the direction perpendicular to the top surface of the bottom shield layer 103, thereby partially etching the protection layer 107, the MR-element-to-be film 106P, and the base layer 105. The protection layer 107, the MR-element-to-be film 106P, and the base layer 105 are thus patterned. The MR-element-to-be film 106P makes an MR element 106 as a result of the patterning.
Next, as shown in FIG. 23, a hard magnetic layer 109P for making bias field applying layers is formed by sputtering on the entire top surface of the laminate obtained by the steps so far, with the mask 108 left unremoved. The hard magnetic layer 109P is made of CoPt, for example. The mask 108 is then lifted off. Portions of the hard magnetic layer 109P remaining after the liftoff make a pair of bias field applying layers 109.
Next, as shown in FIG. 24, a pair of electrode layers 110 are formed on the pair of bias field applying layers 109. The electrode layers 110 are made of a laminate of Au and Ta films, for example. A top shield gap film 111 made of alumina, for example, is then formed on the entire top surface of the laminate. Then, although not shown, a top shield layer is formed on the entire top surface of the laminate.
As disclosed in, e.g., Published Unexamined Japanese Patent Applications (KOKAI) Heisei 11-224411 and 2000-76629, it is known that, when the bias field applying layers are located on both sides of the MR element, regions that may be hereinafter called lower-sensitivity regions develop near ends of the MR element that are adjacent to the bias field applying layers. In these regions, the magnetic field produced from the bias field applying layers limits variations of the direction of magnetization, and the sensitivity is thereby lowered. Consequently, if the electrode layers are located so as not to overlap the MR element, a sense current passes through the lower-sensitivity regions, which thereby reduces the output of the reproducing head. This problem becomes more noticeable as the track width of the reproducing head becomes smaller.
To solve this problem, each of the electrode layers is located such that a portion thereof is laid over part of (hereinafter expressed as “overlap”) the MR element, as disclosed in, e.g., Published Unexamined Japanese Patent Applications (KOKAI) Heisei 11-224411 and 2000-76629. It is possible to reduce Barkhausen noise while preventing a reduction in output of the reproducing head, if the reproducing head has a structure (hereinafter called an overlapping electrode layer structure) in which the bias field applying layers are located on both sides of the MR element, and the electrode layers overlap the MR element, as described above.
Reference is now made to FIG. 25 through FIG. 27 to describe an example of a method of manufacturing a reproducing head having the overlapping electrode layer structure. This manufacturing method has the same steps as those described with reference to FIG. 21 through FIG. 23 up to the step of forming the bias field applying layers 109.
In this manufacturing method, after the mask 108 is lifted off, as shown in FIG. 25, a mask 112 for forming the electrode layers by a lift-off method is formed on the protection layer 107. The mask 112 is made of a photoresist layer patterned by photolithography. The mask 112 has a width smaller than that of the mask 108 shown in FIG. 21 through FIG. 23. For easy lift-off, the mask 112 is formed to have a T-shaped cross section.
Next, as shown in FIG. 26, an electrode-to-be film 113P to make the electrode layers is formed by sputtering on the entire top surface of the laminate. The electrode-to-be film 113P is made of a laminate of Au and Ta films, for example.
Next, as shown in FIG. 27, the mask 112 is lifted off. Portions of the electrode-to-be film 113P remaining after the liftoff make a pair of electrode layers 113. The electrode layers 113 are located to overlap the MR element 106. Then, although not shown, a top shield gap film and a top shield layer are formed in this order on the entire top surface of the laminate. In the reproducing head thus fabricated, the space between the pair of electrode layers 113 defines the optical track width of the reproducing head.
The method of manufacturing the reproducing head shown in FIG. 25 through FIG. 27 requires two masks, namely, the mask 108 for defining the width of the MR element 106 and the space between the pair of bias field applying layers 109, and the mask 112 for defining the space between the pair of electrode layers 113. In typical reproducing heads of the overlapping electrode layer structure, the electrode layers 113 are formed to overlap the MR element 106 by a width of, e.g., 0.1 to 0.2 μm each from the end of each of the bias field applying layers 109 toward the center of the MR element 106 along the width thereof. The two electrode layers 113 are controlled to have the same overlap amount. Hence, alignment of the masks 108 and 112 is of extreme importance.
According to the method of manufacturing the reproducing head shown in FIG. 25 through FIG. 27, however, the MR element 106 and the bias field applying layers 109 are patterned by using the mask 108 while the electrode layers 113 are patterned by using the mask 112. Hence, it is extremely difficult for this manufacturing method to locate the two electrode layers 113 in position as designed. As a result, this manufacturing method presents problems that the actual track width may deviate from a designed value, and that the overlap amount of at least one of the electrode layers 113 may fall below a designed value and the effect of the overlapping electrode layer structure against a drop in output of reproducing heads is thereby hampered.
To solve the foregoing problems, a method of manufacturing a reproducing head as described below is employable. In this manufacturing method, a single mask is used to pattern the MR element, the bias field applying layers and the electrode layers in a self-aligned manner. This manufacturing method will be described with reference to FIG. 28 through FIG. 30. This manufacturing method has the same steps as those described with reference to FIG. 21 up to the step of forming the protection layer 107.
Then, in this manufacturing method, as shown in FIG. 28, a mask 114 for patterning the MR element, the bias field applying layers and the electrode layers is formed on the protection layer 107. The mask 114 is made of a photoresist layer patterned by photolithography. For easy lift-off, the mask 114 is formed to have a T-shaped cross section. Hereinafter, a portion of the mask 114 that is closer to the bottom and smaller in width will be referred to as a root portion.
Then, ion beam etching is performed so that ion beams travel at an angle of 5 to 10° with respect to the direction perpendicular to the top surface of the bottom shield layer 103, thereby partially etching the protection layer 107, the MR-element-to-be film 106P, and the base layer 105. The protection layer 107, the MR-element-to-be film 106P, and the base layer 105 are thus patterned. The MR-element-to-be film 106P makes the MR element 106 as a result of the patterning.
Next, ion beam deposition is performed so that ion beams travel at an angle of 0 to 5° with respect to the direction perpendicular to the top surface of the bottom shield layer 103. The hard magnetic layer 109P to make the bias field applying layers is thereby formed on the entire top surface of the laminate. The hard magnetic layer 109P is made of CoPt, for example. In FIG. 28, the arrows represent ion beams.
Then, as shown in FIG. 29, ion beam deposition is performed so that ion beams travel at an angle of 45° with respect to the direction perpendicular to the top surface of the bottom shield layer 103, with the mask 114 left unremoved. The electrode-to-be film 113P to make the electrode layers is thereby formed on the entire top surface of the laminate. The electrode-to-be film 113P is made of Au, for example. On the protection layer 107, the electrode-to-be film 113P is formed to extend to the vicinity of the root portion of the mask 114. In FIG. 29, the arrows represent ion beams.
Next, as shown in FIG. 30, the mask 114 is lifted off. As a result, the remaining portions of the hard magnetic layer 109P make a pair of bias field applying layers 109, and the remaining portions of the electrode-to-be film 113P make a pair of electrode layers 113. Next, a top shield gap film 115 made of alumina, for example, is formed on the entire top surface of the laminate. Next, although not shown, a top shield layer is formed on the entire top surface of the laminate.
The manufacturing method shown in FIG. 28 through FIG. 30 can solve the problems of the manufacturing method shown in FIG. 25 through FIG. 27. The method of FIG. 28 through FIG. 30, however, has the following two problems.
A first problem will be described with reference to FIG. 31. FIG. 31 is a cross section illustrating the reproducing head manufactured by the method shown in FIG. 28 through FIG. 30 in detail. According to the manufacturing method of FIG. 28 through FIG. 30, a region on top of the protection layer 107 near the root portion of the mask 114 is less exposed to ion beams during the ion beam deposition shown in FIG. 29. Accordingly, as shown in FIG. 31, portions of the electrode layers 113 formed on the protection layer 107 become thinner than the other portions thereof.
Additionally, in the manufacturing method shown in FIG. 28 through FIG. 30, after the protection layer 107 is formed, the laminate is exposed to air in order to form the mask 114 on the protection layer 107. As a result, an upper part of the protection layer 107 is oxidized to form an oxide layer 117. Given that the protection layer 107 is made of Ta, the oxide layer 117 is of TaO2.
For these reasons, the reproducing head manufactured by the method shown in FIG. 28 through FIG. 30 has greater ohmic resistances between the electrode layers 113 and the MR element 106 near the portions of the electrode layers 113 located on the protection layer 107. As a result, this reproducing head has the problem that less current flows between the MR element 106 and the portions of the electrode layers 133 located on the protection layer 107, and the effect of the overlapping electrode layer structure against a drop in output of reproducing heads is thereby hampered.
To solve the above-mentioned problem, it is conceivable to remove the oxide layer 117 near the root portion of the mask 114 by dry etching before the electrode-to-be film 113P is formed. To do so, for easy removal of the oxide layer 117, it is preferable that the root portion of the mask 114 having a T-shaped cross section be made greater in height. However, this causes the following problem in turn.
The problem will be described with reference to FIGS. 32 and 33. FIG. 32 is a cross section illustrating in detail a laminate obtained after the electrode-to-be film 113P is formed by the step shown in FIG. 29. As shown in FIG. 32, the ion beam deposition forms the electrode-to-be film 113P even on the side surfaces of the root portion of the mask 114. Hereinafter, the portions of the electrode-to-be film 113P that are formed on the side surfaces of the root portion of the mask 114 will be referred to as sidewall portions 118.
As shown in FIG. 33, after the mask 114 is lifted off, the sidewall portions 118, which are electrically conductive, remain on the protection layer 107. The sidewall portions 118 can come into contact with the top shield layer to cause a short between the top shield layer and the MR element 106.